The present invention relates generally to apparatus for transmitting signals between two relatively rotatable members. More specifically, the invention involves signal coupler apparatus wherein light signals in a continuous ring in a plane are transmitted between rotatable members without physical contact. The invention offers particular advantages in low cost, multiple channel rotary signal coupler apparatus.
Requirements to transmit electrical power and data across rotary joints have existed for many years. Traditionally, such functions were accomplished with electromechanical sliding contact slip rings. Early slip ring applications, including those for data transmission, typically involved the transmission of appreciable amounts of power. Sliding contacts were well suited to transmission of signals characterized by significant voltages and/or currents.
As technologies requiring the use of slip ring apparatus progressed, the inherent characteristics of electromechanical slip rings began to impose increasingly severe limitations on system performance. More specifically, advancing technologies required increased channel capacity combined with overall size reduction, decreased cross coupling of signals on separate channels, and decreased noise, dead band and power consumption. Attempts to reduce friction levels by reducing contact pressure increased the susceptibility of sliding contacts to lift during vibration, thereby introducing noise and/or signal interruptions. In addition, the lower power levels of contemporary data signals tended to reduce immunity to contact contamination, and consequently to increase the likeihood of signal interruption and/or distortion.
An additional limitation on the use of electromechanical slip rings relates to cost. Precious metals have been used to provide good electrical conduction at low contact forces, and to decrease the amount of maintenance required to keep contact surfaces adequately free of contamination and oxides. Modern plating techniques have helped to reduce the cost of precious metals required in a slip ring assembly. Nevertheless, the precious metal cost for a typical small assembly is several hundred dollars. An additional cost item arises from cleaning and maintenance operations which are routinely necessary several times a year, and which require the services of specially trained and experienced technicians.
one technology which has recently shown considerable promise for overcoming the noted problems involves the transmission of light signals between relatively rotating members. In general, the known devices utilizing this techique comprise a pair of members, one of which is rotatable with respect to the other about an axis of rotation. One of the members carries a light detector which is aligned with a light source carried by the other member. Accordingly, light signals, which may be modulated to convey data or information of interest, are transmitted across the moving/stationary interface without physical contact between the relatively moving members. Typical known forms of such devices are disclosed in U.S. Pat. Nos. 3,401,232, 3,922,063 and 4,027,945 issued respectively to J. S. Goldhammer et al on Sept. 10, 1968, F. A. Marrone on Nov. 25, 1975 and M. L. Iverson on June 7, 1977. Each disclosed device employs a light source and a detector alinged with the axis of rotation. Multiple channels may be provided by arranging light sources and detectors on the axis of rotation and in concentric rings about the axis.
Such arrangements are practical and satisfactory for small numbers of channels. However, for larger numbers of channels the required area perpendicular to the axis of rotation becomes prohibitively large since the area increases approximately as the square of the number of channels. Further, where the light source and/or detector are implemented by spreading out the ends of light transmitting fibers of a fiber bundle into a ring as disclosed in U.S. Pat. No. 4,027,945, the number of fibers required to implement individual channels varies approximately as the square of the radius of the rings. For larger rings, a very large number of fibers becomes necessary, and it becomes increasingly difficult to evenly distribute the light signal around the ring. Further, such construction is generally somewhat unconventional, and the complexities of suitably distributing and maintaining alignment of the fibers contributes to the cost of the device.
The applicant has devised an optical rotary coupler design in which a single optical fiber or small fiber bundle formed in a circular loop is employed to radiate light signals transversely to an axis of rotation. The design is exceptionally simple and capable of implementation with a large number of identical channels axially spaced along the axis of rotation. Further, the design minimizes the necessity for complex distribution and alignment of fibers in an optical fiber bundle. Accordingly, the advantages of using light signals to transmit data across a rotating/stationary interface are provided at minimum cost.